1. Field of the Invention
This invention relates to pharmacological uses of retinoids. More particularly, this invention relates to use of retinoids in treatment of ocular disorders.
2. Description of Related Art
The retinal pigment epithelium (RPE) forms a monolayer of cells beneath the sensory retina that is normally mitotically inactive except when it is participating in retinal wound repair, in which it plays a central role. When wound repair is complete, the RPE usually stops proliferating; failure to do so can result in blinding disorders such as proliferative vitreoretinopathy (PVR) and disciform scarring. For instance, after detachment of the sensory retina, the RPE changes in morphology and begins to proliferate. Multilayered colonies of dedifferentiated RPE cells are formed. Cells then begin to migrate into the subretinal space where they engulf rod outer segments. In some instances cells migrate onto the surface of the retina and form epiretinal membranes. These events have been implicated in the pathogenesis of proliferative vitreoretinopathy, severe scarring occurring in association with macular degeneration, and poor or delayed recovery of vision after retinal reattachment.
Age-related macular degeneration (AMD) is the major cause of blindness in patients over the age of 60 in the United States. Severe loss of vision in patients with AMD is usually due to the development of choroidal neovascularization (CNV). Laser treatment can ablate CNV and help to preserve vision in selected cases not involving the center of the retina; however, the treatment benefit is often transient due to the high rate of recurrent CNV (50% over 3 years and approximately 60% at 5 years) (Macular Photocoagulation Study Group, Arch. Ophthalmol. 204: 694-701, 1986). In addition, many patients who develop CNV are not good candidates for laser therapy because the CNV is too large for laser treatment, or the location cannot be determined so that the physician cannot accurately aim the laser.
Despite these important consequences, little is known about the stimuli involved in RPE dedifferentiation and loss of density-dependent growth control. However, it is known that cultured human RPE rapidly become depleted of retinoids when maintained in media supplemented with fetal bovine serum (FBS) (S. R. Das, et al., Biochem. J., 250:459, 1988). Retinoids have been implicated in cellular differentiation (S. Strickland, et al., Cell, 15:393-403, 1978; T. R. Brietman, et al., PNAS, 77:2936-2940, 1980; and are normally present in high levels in RPE in vivo. Retinoids play a prominent role in visual transduction and therefore their recycling is needed for normal visual function. This recycling occurs through an intimate relationship between the photoreceptors and the RPE. Disruption of this intimate relationship during retinal detachment prevents recycling of retinoids and may be one reason for outer segment degeneration and dedifferentiation of the RPE (P. A. Campochiaro, et al., Invest. Opthalmol. Vis. Sci., 32:65-72, 1991).
Incubation of cultured RPE cells with all-trans retinoic acid (RA) inhibits cell proliferation and promotes a morphologic appearance like RPE in situ (P. A. Campochiaro, et al., supra; J. W. Doyle, et al., Curr. Eye Res. 11:753-765, 1992). All-trans RA and other derivatives of vitamin A (generally referred to as retinoids) affect the growth and differentiation of many cell types (S. Strickland, et al., supra; T. R. Breitman, et al., Proc. Natl. Acad. Sci. USA, 7:2936-2940, 1980). Therefore, retinoic acid or a related molecule may be one of the signals that helps to maintain or re-establish quiescence of RPE and other cells that participate in PVR.
The biological effects of retinoids are mediated through nuclear receptors which are ligand-induced trans-acting factors that bind to DNA response elements, modulating the transcription of genes containing those response elements.
These receptors are divided into two major families, retinoic acid receptors (RARs) and retinoid X receptors (RXRs). For each family there are three separate gene products constituting three subtypes designated .alpha., .beta., and .gamma. (H. G. Stunnenberg, Bio Essays, 15:309-315, 1993). Alternative splicing of mRNA for these subtypes results in different isoforms and even greater diversity. The level of expression of RAR and RXR subtypes differs from tissue to tissue, and differences in activity of subtypes may provide some tissue specificity of retinoid effects (P. Dolle, et al., Nature, 342:702-705, 1989; J. L. Rees, et al., Biochem. J., 259:917-919, 1989). Retinoid receptors up-regulate gene expression by binding to the promoter regions of RA-responsive genes as transcriptionally active RAR-RXR heterodimers (S. Nagpal, et al., EMBO J., 12:2349-2360, 1993) or RXR homodimers (X. Zhang, et al., Nature, 358:587-591, 1992); whereas they down-regulate expression of other genes by antagonizing the effect of transcription factors such as AP-1 (M. Pfahl, Endocrine Review, 14:651-658, 1993; Nagpal, et al., J. Biol. Chem 270:923-927, 1995). AP-1 components c-Jun and c-Fos are products of immediate early genes which are produced in response to the mitogenic signals (e.g., growth factors and tumor promoters) arriving at the cell membrane. The c-Jun/c-Fos complex, in turn, activates the expression of AP-1-responsive genes involved in cell division and cell proliferation (T. Curran and B. R. Franzo, Jr., Cell, 55:395-397, 1988; I. R. Hart, et al., Biochim. Biophys. Acta. 989:65-84, 1989; P. K. Vogt and T. J. Bos, Trends Biochem. Sci., 14:172-175, 1989). On the other hand, retinoids inhibit cell proliferation and induce differentiation (L. J. Guoas, et al., in The Retinoids: Biology, Chemistry and Medicine, eds. M. B. Sporn, et al., Raven Press Ltd., New York, pp 443-520, 1994). Therefore, in the light of the above mentioned observations, retinoid mediated antagonism of AP-1-dependent gene expression provides a basis of their antiproliferative effects. Another level of regulation is provided by differences in ligand-receptor affinities. All trans-RA is the endogenous ligand for RARs, while that for RXRs is believed to be 9-cis-RA (R. A. Heyman, et al., Cell, 68:397-406, 1992; A. A. Levin, et al., Nature, 355:359-361, 1992); however, 9-cis-RA can bind to and activate the RARs as well. 9-cis RA is a stereoisomer of all-trans RA and is generated from all-trans RA in vivo during metabolism (R. A. Heyman, et al., supra).
Proliferative vitreoretinopathy (PVR) is the most common cause of failure following rhegmatogenous retinal detachment surgery. Despite meticulous surgical membrane removal and the use of tamponades such as silicone oil (SiO), failure occurs in a large number of cases due to difficulty with complete removal and continuous growth of the membranes. To date, the goals of surgery for PVR are to relieve traction by removal of epiretinal membranes and portions of foreshortened retina when necessary, surround all retinal breaks with retinopexy, and inject gas or silicone oil into the vitreous cavity to provide retinal tamponade for a sufficiently long period of time that all retinal breaks are sealed. Using these techniques, retinal reattachment is achieved in 35-45% of eyes with PVR with one operation, and in up to 71% of eyes with multiple operations. However, with each operation the visual prognosis worsens (Silicone Study Group, Silicone Study Report No. 2., Arch Ophthalmol,. 110:780-792, 1992). The major cause of failure is reproliferation with regrowth of epiretinal membranes resulting in traction and recurrent detachment. Therefore, prevention of reproliferation is a major goal in the treatment of PVR.
A number of experiments have been reported using different antiproliferative agents, such as daunomycin, alone or in combination with vitreoretinal surgery. Retinoids are lipid soluble, as most antiproliferative agents are not. All-trans RA dissolved in SiO was tested in a rabbit model of PVR, and produced a significant and lasting reduction in cellular proliferation. At doses of 2 to 20 .mu.g/ml SiO in 3 kilogram rabbits no histological evidence of retinal toxicity was found, and the rate of retinal detachment was decreased from 100% in untreated controls to 44.5% in treated rabbit eyes at 8 weeks (J. J. Araiz, et al., Invest. Ophthalmol. Vis. Sci., 34:522-30, 1993). This mode of retinoid delivery is suitable for patients with advanced PVR in whom silicone oil is often used, but is not applicable for use in patients with early or less severe PVR or those patients at high risk for PVR after retinal reattachment.
The rabbit model has also been used to test sustained delivery of RA from microspheres of biodegradable polymer in treatment of PVR (G. G. Giordano, et al., Invest. Ophthalmol. Vis. Sci., 34:2743-2751, 1993). Filling the vitreous cavity with a suspension of biodegradable microspheres into which a total of about 100 .mu.g of all-trans RA was incorporated significantly decreased tractional retinal detachment (TRD) in eyes treated with gas compression vitrectomy and fibroblast injection. Toxicity was limited to localized areas of inflammation felt to represent a foreign body reaction.
Retinoic acid, its geometric isomer 13-cis-retinoic acid, and synthetic derivatives have numerous biological effects in several tissues. Retinoids are currently used as the first line treatment for acute myelogenous leukemia (R. P. Warrell, Jr., et al., N. Engl. J. Med., 324:1385-1393, 1991), as adjuvant therapy for several types of metastatic carcinomas (K. Dhingra, et al., Invest. New Drugs, 11:39-43, 1993; S. M. Lipman, et al., J. Natl. Can. Inst., 85:499-500, 1993), and for treatment of psoriasis and other skin disorders. While these varied effects of retinoids provide multiple clinical applications, they are also the basis for undesired effects and toxicity that can impede the treatment of any one particular disorder. For instance, 13-cis-retinoic acid is associated with teratogenic effects when administered to pregnant females of child-bearing age. Thus, the need exists for additional synthetic retinoid analogs that avoid these toxic effects for use in treatment of PVR. Also, understanding the mechanism by which retinoids exert their antiproliferative effect in RPE cells will enable the development of new and adjunctive therapeutic agents for PVR and related diseases.